In paper manufacture chemical additives have a pivotal role in enhancing the functionality of the paper product. A particularly desired functional property is dry strength, which is related to the structure of the paper web. Dry strength originates from hydrogen bonds between the fibers formed during the drying process. Additives for increasing or preserving dry strength belong to three categories: Additives enhancing fiber-fiber bonding; b) Additives enhancing fiber-fiber bond formation; and c) Additives improving fines retention and drainage.
Starch, both native and modified, is by far the most commonly used dry strength additive. In contrast to modified (cationic) starches used as dry-strength improving additives at the wet-end of the paper making process native starch lacks affinity towards cellulose fiber. While high starch addition rates are desired they result in incomplete starch adsorption on the fiber and thus in reduced efficiency, in operating problems caused by high levels of non-adsorbed starch re-circulating in the process, and in the difficulty to further increase the starch retention level. These effects are evident for cationic starch derivatives. Native starch can be applied by spraying methodology, for example in between layers on the paper machine. Alternatively, native starch can be applied by a coating process at the dry end of the paper machine.
U.S. Pat. No. 4,088,530 discloses that synthetic polymers, polyacrylamide, have been used as dry-strength additive.
WO 2006/079512 discloses a method of enhancing the mechanical properties of cellulose-based materials in form of a multi-component multi-step process based on xyloglucan.
Sodium carboxymethyl cellulose (CMC) is an anionic, water soluble polymer with no inherent affinity towards cellulose fibers. In order to create bonding between CMC and cellulose material different methodologies have evolved. These methods are however not efficient enough for general use in paper making but are restricted to the production of specialty papers, which can bear their high cost: Alum has been used as a cationic linker between CMC and cellulose fibers (Watanabe et al, Tappi J. 2004, 3, 15): U.S. Pat. Nos. 5,061,346 and 6,294,645 and patents cited therein relate to systems for imparting dry-strength to paper, a cationic component being sandwiched between the fiber and CMC. The cationic component is a polyamidoamine that has been treated with an epoxyhalohydrin. This methodology has been reviewed by Wågberg (Nord. Pulp. Pap. Res. J., 2000, 15, 586). Another approach to improve the dry strength of paper by co-crystallizing CMC and cellulose fibers before or after beating, that is before the fibers reach the paper machine is disclosed in PhD thesis by M Blomstedt (“MODIFICATION OF CELLULOSIC FIBRES BY CARBOXYMETHYL CELLULOSE EFFECTS ON FIBER AND SHEET PROPERTIES” Helsinki University of Technology, 2007). However, long reaction times, high pH, and high CMC loadings make this one-step process industrially disadvantageous.
There is no efficient one-step process disclosed in the art by CMC can be applied to paper or other fibrous cellulose products as a strengthening enhancer, and which is compatible with the process restrictions set by paper forming machinery.